1. Field of the Invention
The present invention relates to a function generation circuit, employed for a temperature-compensating quartz oscillation circuit that employs a quartz oscillation frequency, an electronic apparatus and a communication apparatus.
2. Description of the Related Art
Recently, the demand for portable electronic apparatuses has increased remarkably, and for such electronic apparatuses, compact, accurate quartz oscillators that generate reference clock signals are requisites.
As the temperature characteristics, the oscillation frequency of a quartz oscillator included in a quartz oscillation apparatus has the third and first order temperature elements due to a crystal unit that is employed for a quartz oscillator. Specifically, when as shown in FIG. 17A the horizontal axis represents an ambient temperature Ta and the vertical axis represents an oscillation frequency f, the characteristic of the oscillation frequency f of the quartz oscillator when temperature compensation is not performed is substantially as shown by a cubic curve 101A, where there is a shift of about 10 ppm to 30 ppm between a relative maximum value and a relative minimum value. In this case, the ambient temperature Ta is defined as being about −30° C. to +80° C. Therefore, when as shown in FIG. 17B the horizontal axis represents the ambient temperature Ta and the vertical axis represents a control voltage Vc, an ideal control voltage curve 102A is generated. And when as shown in FIG. 17C a voltage is applied to the quartz oscillator, df/dTa=0 is obtained, and the oscillation frequency f does not substantially depend on the temperature.
Presently available is an example temperature compensation method whereby a varactor diode (=a variable capacitance diode), which is a frequency adjustment element, is connected to a quartz oscillator, and whereby, to stabilize an oscillation frequency, a control voltage having the third and the first temperature characteristics, for compensating for the temperature characteristic of the quartz oscillator, is applied to the varactor diode.
Actually, it is technically difficult to generate a control voltage Vc having the ideal temperature characteristic shown in FIG. 17B, and generally, a control voltage having a pseudo third temperature characteristic is generated, by employing various methods, in order to provide temperature compensation for the quartz oscillation apparatus.
FIG. 18 is a block diagram showing the configuration of a conventional quartz oscillation apparatus that includes a temperature compensation function. According to the temperature compensation method of this quartz oscillation apparatus, the third and first temperature characteristics of a quartz oscillator are both divided to obtain a plurality of temperature domains, and for each temperature domain, a crossover approximation is obtained by employing, as a linear temperature line, a voltage level that serves as a temperature function.
Specifically, for the individual voltage line areas, the temperature domains obtained by dividing the temperature characteristics, the temperature coefficients (factors of proportionality) of linear temperature lines in the temperature domains, and voltage values at the normal temperature along the linear temperature line are stored in a memory circuit 111A shown in FIG. 18, and voltage line data corresponding to an ambient temperature detected by a temperature sensor circuit 112A is selectively read from the memory circuit 111A. An amplification circuit 113A generates a predetermined control voltage based on the control voltage data that is read, and applies the generated control voltage to a voltage-controlled quartz oscillator 114A. In this manner, temperature compensation for the quartz oscillation apparatus is provided, and the oscillation frequency is stabilized.
Patent Document 1: JP-A-8-288741
However, since the conventional quartz oscillation apparatus equipped with a temperature compensation function employs A/D conversion to obtain a crossover approximation for generation of a control voltage used for temperature compensation, a quantized noise occurs, and essentially, a frequency skipping problem can not be avoided. Further, since a clock signal generation circuit is required, there is a problem with the mixing of clock noise, and a problem in that, when power is on, it takes time for an oscillated frequency to be stabilized because of a time constant for a sample holding circuit 115A.
Furthermore, in a process for measuring and adjusting a temperature characteristic, the temperature characteristic of a frequency oscillated by a quartz oscillation apparatus is measured by discretely changing the ambient temperature, and temperature compensation for the quartz oscillation apparatus is performed. Therefore, an adjustment error occurs. In order to reduce this error, the number of domains obtained by dividing the temperature characteristic must be increased; however, in this case, the memory volume of the memory circuit 111A is increased.
While taking these problems into account, one objective of the present invention is to provide a function generation circuit that enables the attainment of an accurate TCXO (temperature-controlled quartz oscillator), for which the basic frequency fluctuation error is ±0.5 ppm within a large temperature domain.